This invention relates to a hydrostatic two-circuit steering system for an automotive vehicle or the like of the type in which the steerability of the vehicle is maintained by one circuit in the event failure of the other circuit occurs.
In such steering systems, the hand operated and metering pumps act as manually driven pumps in the absence of auxiliary fluid power. When auxiliary power is available, such pumps act as metering pumps to control the supply of pressure fluid to the servomotors in accordance with the displacement of the manual operating mechanism. In such prior known steering systems of the two-circuit type, rotary pump gear elements of reversible flow, positive displacement pumps serve as the hand operated and metering pumps. When the control valves of the system are shifted by the operating mechanism, the outlet sides of the pump elements are always connected to one pressure chamber of one of the two servomotors which are mechanically coupled to each other. A high operating pressure from a pressure source acts on the inlet sides of the two pump elements while the pressure vented chambers of both servomotors are connected to a fluid discharge line. Thus, in all operative positions of the control valves, separate hydraulic circuits are established each of which include one pump element and one pressure chamber associated with one of the two servomotors. In the neutral or center position of the control valves, both pressure chambers of both servomotors as well as the connections to both pump elements are blocked. However, the two hydraulic circuits are not completely separated. Therefore, as a result of a break in a critical line connection to a servomotor, the steerability of the vehicle is maintained for only a limited period of time, or until the operating fluid is drained at the break location.
Although duplication of prior known steering systems has been suggested including use of separate pumps and separate fluid reservoir tanks, the mere establishment thereby of two separate circuits creates problems in view of the coupling of the pumps with a common operating mechanism for drive in synchronism with each other. As a result of production tolerance variations, the volumetric outputs of the pumps deviate from each other. Further, the volumetric flow to and from the two servomotors differs from each other because of non-uniform piston surfaces to disturb the symmetrical relationship between the servomotors from an operational standpoint. Consequently, there is an operational phase differential created as a result of which hydraulic and mechanical interference between both circuits occurs.
It is therefore an important objective of the present invention to provide a hydrostatic steering apparatus for vehicles having two completely separate hydraulic circuits each of which include a pressure source and a servomotor with facilities to avoid functional hydraulic interference between the two circuits.